Well i don’t see issue here since, source can be available it does not mean it can’t be distributed in binary form. 2nd marketplace support for plug-ins is coming, so all Xamarin would need to do is place binaries there… but free of charge as EULA states which from what i understand is a issue for Xamarin
Those are build scripts used for UBT which let you modify compile process, aditionally all code build tools are written in C#, but they are technicaly separate software, not real part of the engine, editror just run them.